For the purpose of improving both the light power and the energy efficiency of vehicle headlights, current technical developments are leading to replacing filament lamps with discharge lamps. Unlike conventional bulbs, which were designed to be connected directly to the battery of the vehicle, these new lamps require high voltages, alternating or direct, depending on the operating mode, in order to create and maintain the electrical discharge in the gas.
These high voltages, specific to each type of lamp (around 25 Kv for lighting a xenon lamp), are produced from the onboard voltage by a supply module, which also provides the power regulation, known by the term “ballast”.
An example of a ballast adapted to automobile applications is described in U.S. Pat. No. 5,036,256.
In accordance with U.S. Pat. No. 5,036,256, when the discharge lamp is started up, the ballast produces, from the 12 V voltage of the battery, a high DC voltage of 500 to 700 V and a pulse of 25-30 Kv, and then a maintenance voltage of around 90 V. To do this, use is made of a chopping supply functioning at 50 kHz.
In general terms, though discharge lamps have certain advantages over filament lamps, their functioning could be the source of significant radio interference should they be improperly used.
Problems posed by the electromagnetic compatibility (EMC) of this type of equipment are known from the prior art. It is known that the various items of onboard equipment, based more and more on complex electronic systems, may be interfered with, but also that the car radio may be subject to interference, in a way that is obviously much more perceptible to the driver and passengers in the vehicle.
A partial solution to these problems is described in the French patent application FR2703555.
In accordance with FR2703555, the measure provided for limiting the diffusion of electromagnetic radiation consists of producing headlights of compact design by arranging the lamp and at least part of the ballast in the housing. A filtering circuit comprising the coils in series with the lamp and capacitors in parallel is integrated in the connector. However, the radio interference reduction measure taught by FR2703555 concerns only interference created by the arc of the discharge lamp itself, and is completely silent on the noise generated by the chopping supply generally forming part of current ballasts.
Technological advances and the search for better efficiency are leading towards an increase in the operating frequency of the ballast.
Chopping frequencies above 100 kHz, 200 kHz or even 300 kHz are currently used, which corresponds to the “long wave” range in radio broadcasting. Under these circumstances, reducing the radio interference emitted becomes a serious constraint for the ballast and automobile equipment manufacturers.
The European standard CISPR-25, issuing from the work of the Special International Committee on Radio Interference, relating to the “limits and methods of measuring the characteristics of radio interference for the protection of receivers used on board vehicles” defined a certain number of constraints for frequencies above 150 kHz. Thus this standard recommends a maximum interference level in pipe mode of 60 dBμV in the frequency band from 150 kHz to 300 kHz on the electrical harness of the vehicle for equipment in class 4.
The general question of the optimization of the EMC in chopping supplies has been studied in detail by electronics engineers. For example, in an application note published in 2003 by the company Texas Instruments and entitled “Understanding and Optimising Electromagnetic Compatibility in Switchmode Power Supplies”, the authors B. Mammano and B. Carsten review the various known methods of reducing radio interference emitted, whether in radiated mode or pipe mode, and, in the latter case, both in common mode and differential mode.
The basic method for reducing noise in differential mode consists of inserting, between the chopping supply and the electrical system, a filter comprising a choke of several tens of Mh in series and a capacitor of several thousands of Mf in parallel. However, the stray capacitance of such a choke and the stray inductance of a capacitor with such a value, necessarily of the “electrochemical” type, make a filter as simple as this inoperative.
In order to reduce the effects of the equivalent inductance of the capacitor, several capacitors of lower value are associated in parallel, and, in order to limit the stray capacitance of the choke, it is recommended to use a special winding with a single layer.
According to the teaching of this application, LC circuits can also be used in additional stages, damping them as required by means of RC circuits in order to avoid resonances which might lead to overvoltages or self-oscillations.
However, the latter methods have the drawback of making the scheme of the basic filter more complex. The result is an increase in the number of components to be installed, which is prohibitive for a ballast circuit which, as has been seen, must be as compact as possible, without of course mentioning the problem of additional cost.